1+2+3=I
2+3+4=N
3+4+5=V
4+5+6=E
5+6+7=x
6+7+8=y
The last 2 lines do not fit the previous four.
What replacements (x,y) would?
I was tempted to go for INVENT INVEST or INVERT, but decided rather not to give hints that make it too obvious and
Just leave the results 6,9 …etc
So: the same SIMPLE LOGIC that produced I N V. E SHOULD CREATE THE ADDITIONAL 2 letters.